Illustrated, small slim quarto, pp (16), paperback, a good clean copy. SCARCE. Postage will be reduced. [In 1821, Sir William Congreve patented a process he described as compound-plate printing, with the aim of countering rising levels of banknote forgery. Congreve’s scheme was outlined in an 1820 pamphlet. It comprised two main processes. First, the banknote printing plate would consist of two parts: the upper one would have shaped holes in it, into which identical protuberances in the lower one fitted. Once interlocked together, the two parts would be engraved simultaneously with complex geometric patterns that overlapped across both of them. Secondly, when in use on a specially devised printing press, each plate would be inked in a different colour before being brought together to produce a two-colour image from a single impression. So, the process was ‘compound’ in two senses: it used pairs of plates to make two-colour images but, most importantly, its strength came from employing both geometric patterns and multiple colours in such a way that the two elements could not be used separately to achieve the same result. In Congreve’s words, the process was ‘indivisible’.]